The present invention pertains to an optical apparatus for visual field testing. In particular, the present invention relates to an optical apparatus for visual field testing using a back projection system.
A visual field tester is an apparatus that is used to test, among other things, the peripheral vision of a human eyexe2x80x94such an apparatus has been known in the art for many years. Test results from visual field testers are used to diagnosis diseases that cause degradation of vision sensitivity. For example, a Standard Automated Perimeter (SAP), one of the most accepted of such testing apparatus, typically performs brightness contrast sensitivity tests over a large visual field.
In prior art visual field testers used to perform a contrast sensitivity test, it is common to include a hemispherical projection surface and a stimulus optical projection system. In a typical such visual field tester, the hemispherical projection surface is uniformly illuminated (for example, using a white light source) to provide a constant and uniform background illuminationxe2x80x94the aim is that the hemispherical projection surface be a Lambertian illumination surface (i.e., a surface upon which brightness is constant over different viewing angles). In a typical such visual field tester, the stimulus optical projection system presents stimuli (typically in the form of a circular spot) at selected points on an internal surface of the hemispherical projection surface. For example, this is done by sequentially flashing images of light sources on the internal surface of the hemispherical projection surface where the position and brightness of the stimuli are specified by a computer implemented algorithm. In use for testing, a patient""s eye is placed at, or close, to a center of a surface of a volume enclosed by the hemispherical projection surface, and the patient is asked to respond to the stimuli by pressing, for example, a mouse button. Then, the contrast sensitivity of the patient""s visual field is mapped by changing the brightness and position of the stimuli on the constant, uniform background illumination, and recording the patient""s response thereto. However, such prior art devices have a drawback in that they are bulky and expensive. In particular, the radius of the hemispherical projection surface is typically set to about 30 cm to enable the patient to see the stimulus comfortably (i.e., without straining the patient""s test eye).
U.S. Pat. No. 5,870,169 (the ""169 patent) discloses a visual field tester that utilizes a hemispherical surface in an alternative manner to that described above. Specifically, instead of projecting a stimulus onto an internal surface of a hemispherical projection surface from the patient""s side, as disclosed in the ""169 patent, a rear projection device is used to project a stimulus onto an external surface of a hemispherical projection surface. In this case, the hemispherical projection surface is comprised of a transparent material, and the patient can see the stimulus when it is viewed from the interior surface of the hemispherical projection surface. The apparatus in the ""169 patent provides a stimulus having an improved shape and brightness uniformity over the apparatus""s 72-degree visual field when compared with the shape and brightness uniformity of prior visual field testers. Since there are moving mechanisms on the back of the projection surface, the background illumination is provided from the front side of the projection surface. Therefore, the projection surface has to be coated with an absorbing material to reflect the background illumination from the front side. However, due to absorption by the absorbing material, the maximum brightness of the stimulus is reduced.
Another visual field tester is disclosed in U.S. Pat. No. 5,046,835 (the ""835 patent). As disclosed in the ""835 patent, the size of a visual field tester can be reduced by using a cupola-less optical system. In particular, the ""835 patent discloses a direct viewing optical system that projects a stimulus directly into a patient""s eye. To do this, the optical system images a light source onto an intermediate image plane of an eyepiece (to serve as a stimulus), and separately images light output from a diffused light source onto the intermediate image plane (to serve as a uniformly illuminated background). Then, the stimulus and the background are combined through a beamsplitter, and projected onto the patient""s retina. In use, the patient views a test field through large field of view (60 degrees), long working-distance eyepieces. Because the apparatus disclosed in the ""835 patent does not utilize a hemispherical projection surface, the size of the apparatus is significantly reduced. However, the cost of a large field of view, long working-distance eyepiece is increased due the aperture size of the lens.
U.S. Pat. No. 6,139,150 discloses a visual field tester that uses a retro-reflector array to eliminate use of an eyepiece. As a result, the visual field tester has reduced cost and complexity when compared with the above-described apparatus. However, it is difficult to manufacture a retro-reflector array having a quality that is as high as is required to perform a visual field test.
In light of the above, there is a need in the art for further visual field testers that can, for example, provide an SAP test in a cost-effective way.
One or more embodiments of the present invention advantageously satisfy the above-identified need in the art, and provide a back projection visual field tester. Specifically, one embodiment of the present invention is a visual field tester of a patient""s eye that comprises: (a) a projection screen; (b) a stimulus projection system that projects a light stimulus onto a first side of the projection screen; (c) a background projection system that projects a background light onto the first side of the projection screen; and (d) a lens system disposed on a second side of the projection screen that directs light transmitted through the projection screen to the patient""s eye.